The goal of this project is to identify how workplace factors (lifted load, posture) and gender influence the loading tolerance of the spine during lifting exertions. Spinal buckling tolerance describes the loading tolerance at the threshold of instability and subsequent tissue injury. Buckling tolerance is controlled by neuromuscular control of spinal stability. The proposed research is a continuation of R01 AR46111 where we demonstrated: 1) stiffness of active muscles and reflexes are both necessary to maintain spinal stability;and 2) these factors are influenced by trunk extension force, posture, and gender. These indicate that gender and workplace factors influence injury tolerance (stability and associated buckling tolerance). To understand how workplace factors influence spinal stability and buckling load we will complete three specific aims. SPECIFIC AIM 1 will measure how reflex force and stiffness of active muscles are modified by load, posture and gender during voluntary trunk extension exertions to simulate manual materials handling / lifting tasks. These data will be used to calibrate a subject-specific biomechanical analysis of spinal buckling load (calibrated parameters of intrinsic muscle stiffness gain and reflex gain). SPECIFIC AIM 2 will measure the torso kinetics, kinematics and EMG in separate dynamic lifting exertions to simulate low- risk and high-risk manual materials handling / lifting tasks. These data will be analyzed using the calibrated, subject-specific biomechanical analysis of stability to output buckling tolerance of the spine as a function of lifted load, posture and gender. SPECIFIC AIM 3 will compare empirically measured torso stability with the data computed in specific Aim 2. Nonlinear dynamic analyses (Floquet multipliers) will be determined from data recorded during small torso disturbances applied during trunk extension exertions similar to conditions recorded in Specific Aim 2. The roles of intrinsic muscle stiffness and reflex contributes to spinal stability will be investigated separately. Effects of load, posture and gender will be examined. This research is designed to provide insight into the underlying pathophysiology of lifting-related musculoskeletal injury that can be used by ergonomic experts to investigate improved MMH task design and safer gender inclusion in the workplace.